The invention relates to the use of high-speed data transmission in a mobile network and especially to a handover between mobile switching centers.
FIG. 1 shows the elements that are essential for the invention in a cellular mobile system. Mobile stations MS communicate with base transceiver stations BTS servicing radio cells Cn. The base transceiver stations are connected via base station controllers BSC to mobile switching centers MSC. A subsystem controlled by one BSC (comprising the BTSs controlled by the BSC and other elements of the mobile network not shown in the figure) is called a base station subsystem BSS. The interface between an MSC and a BSS is called an A-interface.
The MSC handles the coupling of incoming and outgoing calls. It performs similar tasks as an exchange of a public switched telephone network PSTN. In addition to these tasks, it also carries out functions that are only characteristic of mobile telephone communication, such as location management of subscribers, together with the subscriber registers in the network. The subscriber registers of the GSM system include at least a home location register HLR and a visitor location register VLR that are not shown in FIG. 1.
The GSM system is a time division multiple access (TDMA) system where the communication on the radio path takes place on a time division basis in successive TDMA frames each of which consists of several time slots. In each time slot, a short information packet is transmitted in the from of a radio-frequency burst that consists of a number of modulated bits. In addition to traffic channels transmitting speech and data, the GSM system also utilizes control channels for performing signaling between a base transceiver station and mobile stations.
Telephone traffic between MSCs is transmitted for example via a PSTN. Also, signaling data is transmitted between MSCs via a so-called MAP (Mobile Application Part) connection. The MAP procedure is defined in GSM standard 09.02 of the ETSI.
When an MS moves from one cell to another, a handover is performed in the system. There are several types of handovers depending of which elements of the mobile system participate in the handover. When the handover takes place within the area of the same base station controller BSC-x, it is called an inter-BTS handover. When the handover takes place within the area of the same mobile switching center MSC-x, it is called an intra-MSC handover. When the MS moves from the area of the first mobile switching center MSC-A to the area of a second mobile switching center MSC-B, the handover is called an inter-MSC handover.
According to the known technology, an inter-MSC handover takes place in the following manner. When an MS that has started a call in the area of MSC-A moves to a cell within the area of another mobile switching center MSC-B, the first mobile switching center MSC-A derives the address of MSC-B from the data of the target cell and establishes an MAP connection to MSC-B. Over the MAP connection MSC-A transmits to MSC-B data about the cell to which it should allocate resources (such as a radio channel and corresponding connections to the fixed network). After the resource allocation, a handover number HON is allocated. Data about the channel and the HON is transmitted to MSC-A via the MAP connection. After MSC-A has obtained the HON, it establishes a connection via the PSTN by means of the HON. When the PSTN connection has been set up, MSC-B transmits an acknowledgment to MSC-A. MSC-A then transmits to MSC-B a handover command (HANDOVER_COMMAND) which is forwarded to the MS. After a successful handover the MS transmits a HANDOVER_COMPLETE message in the uplink direction. This data is forwarded to MSC-A, which releases the earlier resources. The handover command is described in GSM recommendation 04.08, version 4.5.0, June 1993, section 9.1.15.
FIG. 2 shows steps and signaling messages related to a handover between a first mobile switching center MSC-A and a second mobile switching center MSC-B. It should be noted, however, that during a handover also other messages, which are not shown in the figure for the sake of clarity, are transmitted in addition to the messages described herein. 1: An MS transmits the results of the neighbouring cell measurements (MEAS_REPORT) to the servicing base station system BSS-A. 2: BSC-A determines the need for a handover to a cell of the new base station system BSS-B, for example on the basis of radio path criteria. 3: BSS-A transmits a request for a handover HANDOVER_REQUIRED to the servicing mobile switching center MSC-A. 4: MSC-A transmits a handover request PREP_HANDOVER_REQ to the new mobile switching center MSC-B. 5: MSC-B transmits a request for a handover HANDOVER_REQUEST to BSS-B, more precisely to the BSC of the system, the request asking the new base station system BSS-B to provide the required service. 6: If there are resources available, BSS-B transmits a message HANDOVER_REQUEST_ACKNOWLEDGE to MSC-B. 7: MSC-B transmits to the servicing center MSC-A an acknowledgment message PREP_HANDOVER_RESP. 8: MSC_A transmits to the servicing base station system BSS-A a HANDOVER_COMMAND message. 9: BSS-A transmits to the MS a HANDOVER_COMMAND message. 10: The MS can now start communicating in the new cell in the base station system BSS-B. 11: The MS transmits to BSS-B an acknowledgment HANDOVER_COMPLETE: 12: BSS-B transmits a corresponding acknowledgment HANDOVER_COMPLETE to MSC-B. 13: MSC-B transmits to MSC-A an acknowledgment message SEND_END_SIGNAL_REQ informing MSC-A that the MS has moved to the new base station system in MSC-B. 14-15: The resources allocated to the call are finally released in the old base station system BSS-A.
A standard GSM connection and a handover employed therein only relate to a normal speech or data connection that maintains a connection via only one subchannel, such as a time slot. In high-speed data transmission, an MS that requires data transmission of higher rate than what one traffic channel can provide for transmitting user data is allocated a channel or time slot configuration comprising two or more time slots from the same or different frame on the same or different frequency by means of so-called multi-slot access. It is not essential for the present invention which multi-slot access is used. An example of multi-slot access where the present invention is applicable is disclosed in Finnish patent applications 942190 and 942191 by the same applicant. In these applications, a high-speed signal is multiplexed to several channels (time slots) having a lower speed, it is thus transmitted over the radio path and demultiplexed in the receiver back into one signal. If this technique is applied to the above-described handover according to known technology, the following changes occur:
In step 6, the acknowledgment message HANDOVER_REQUEST_ACKNOWLEDGE contains data about the allocated data rate and a description of the allocated time slots. In step 7, the acknowledgment message PREP_HANDOVER_RESP contains data about the allocated data rate and a description of the allocated time slots, in addition to the content specified in the GSM standards. In steps 8 and 9, the HANDOVER_COMMAND contains data about the allocated data rate and a description of the allocated time slots. In step 10, the mobile station uses the allocated channel configuration and data rate and the allocated time slots.
However, the aforementioned patent applications do not discuss in detail how a data connection utilizing several time slots would allocate handover numbers to different subchannels during a handover.
Problems may also occur during a handover in high-speed data transmission if the new cell cannot provide a sufficient number of channels. Problems occur if a mobile station operates with a high data rate and the new cell is not able to provide after the handover a data rate equal to that supplied by the previous cell.